Pneumatic braking systems employing compressed air are well known, particularly, in the rail transportation industry, e.g., freight and passenger trains. Pneumatic braking systems are also employed in many road transport vehicles, such as heavy trucks.
Typically, in a train, a so-called "brake pipe" carrying compressed air runs from railcar to railcar for the length of the train. Such a "brake pipe" is made up of a series of individual compressed air lines running the length of each railcar which are coupled end to end during the formation of the train. During a braking operation, the braking force that is applied to the wheels of each of the railcars is typically dependent upon the pressure in the brake pipe. This pressure is controlled by the engineer or other operator.
The weight of a railcar (or any rolling stock for that matter) can vary significantly depending upon whether or not it is loaded. For example, a modern lightweight aluminum hopper railcar can weigh as little as 44,000 lbs. yet have a gross loaded weight of up to 286,000 lbs., meaning a "gross to tare ratio" of about 6.5 to 1. It is well known in the rail transportation industry that, if a train is made up of both loaded and empty railcars, then a high braking force which would be appropriate for the loaded railcars will tend to cause the wheels of the empty cars to slide (i.e., skid). Such wheel sliding on empty railcars can be significantly reduced by lowering the braking force applied by the pneumatic braking system to the empty railcars. To this end, so-called "empty/load" brake equipment has been developed and is well understood by those of ordinary skill in the pneumatic brake field.
Normally, a railcar will include a wheeled "truck" portion, which contacts the track, and a "railcar body" portion, which is mounted on the truck by a resilient suspension system, e.g., springs. In an empty condition, the railcar body rides relatively high above the truck. When the railcar is loaded, the suspension is compressed and the railcar body moves closer to the truck. One well known type of widely used empty/load brake device utilizes this phenomenon to operate substantially "automatically" by employing a sensor arm which extends to determine the truck to railcar body clearance and which then reduces the braking force if the sensor arm is able to extend beyond a determined point due to a relatively large truck to railcar body clearance indicating the associated railcar as being in a substantially empty condition.
Such automatic empty/load brake devices include the well known units manufactured by Westinghouse Air Brake Company (also referred to herein as "WABCO") under the designations "SC1" and "ELX". The term "SC1/ELX-Type" brake devices (or equipment or units) is used herein to refer collectively to the SC1 and ELX units manufactured by Westinghouse Air Brake Company, as well as to other automatic empty/load brake devices which function in a similar manner to reduce braking force dependent upon the movement of a sensor arm (or sensor member).
SC1/ELX-Type automatic empty/load brake devices can be mounted on either the truck or the railcar body. Most typically, however, they are mounted on the railcar body. When the brakes are applied, the sensor arm moves toward the other of the truck and the railcar body. The distance which the sensor arm is able to traverse unimpeded is a measure of the truck to railcar body clearance and, hence, the loaded or unloaded condition of the railcar.
If the sensor arm is able to traverse past a particular "transition position", the railcar is determined to be in an unloaded (or empty) condition. The automatic empty/load brake device then operates to reduce the pressure supplied to the brake cylinders (and therefore the braking force) for the associated railcars. In so-called "dual capacity" empty/load brake equipment, the reduction (or proportioning) of the brake cylinder pressure for a railcar determined to be empty is a set proportion of the full unreduced brake cylinder pressure for a loaded railcar. Typically, the brake cylinder pressure for a railcar determined to be empty is in the range of either 50% or 60% of the normal brake cylinder pressure for a loaded railcar. However, one of the aforementioned ELX units proportions the unloaded brake cylinder pressure at 40%.
Conversely, if the sensor arm is not able to move past the transition position due to its contacting the other of the truck and the railcar body, the railcar is then determined to be loaded. The automatic empty/load brake device then operates to apply the full brake cylinder pressure as determined by the brake pipe pressure.
It will be understood, therefore, that the "automatic" functioning of such an automatic empty/load brake device is dependent upon a proper sensing of the truck to railcar body clearance.
However, the total possible truck to railcar body clearance (or "available spring travel") can vary depending upon the national standards of the particular country in which the equipment is being operated. In this regard, while the particular SC1/ELX-Type automatic empty/load brake devices manufactured by Westinghouse Air Brake Company referred to above are designed to function properly with standard U.S. rolling stock, it has been found that such SC1/ELX-Type brake equipment may not function as well as could be desired in a number of non-U.S. countries where the available spring travel may be significantly less than that adopted as standard for U.S. rolling stock. Possible examples of such countries include, but are not limited to, Egypt, Turkey, England, certain European regions and other UIC-type territories.
Additionally, there are a number of countries and regions which have adopted so-called "manual" empty/load brake devices as a standard. In a manual empty/load brake device, the loaded or empty condition of the railcar is not sensed automatically. Instead, such manual devices require the operating personnel to manually shift the braking adjustment device for each railcar or group of associated railcars.
Therefore, at present, SC1/ELX-Type brake equipment may be underutilized, either in countries where the adopted standard provides for too little available spring travel or in countries is having a standard which requires a manual shifting of empty/load braking devices.
Currently, the various SC1/ELX-Type automatic empty/load brake equipment units manufactured by Westinghouse Air Brake Company which employ such a sensor arm to determine the loaded or unloaded condition of railcars include an ELX-U unit, an ELX-B unit and an SC1 unit. In the ELX-U unit, a sensor unit, a proportioning unit and an air equalizing reservoir are all provided as an integral one-piece assembly. In the ELX-B unit, a sensor unit and a proportioning unit are combined integrally, while an equalizing reservoir is separately mounted. Finally, in the older style SC1 unit, a sensor unit, a proportioning unit and an equalizing reservoir are all mounted separately from one another.